Environmental seasonality and incremental growth rates of beaver (Castor canadensis) incisors: Implications for palaeobiology

Periradicular bands are incremental structures seen on the external dentin surfaces of rodent incisors. They correspond to circadian developmental rhythms in dentin growth and provide a record of the growth rate of the incisor along its longitudinal axis. In the present study, we investigate the relationship between daily environmental temperatures and longitudinal growth rates of incisors in 40 Recent beavers (Castor canadensis). We also look for trends in growth rates that reflect precipitation or photoperiod. The goal is to explore the potential of using longitudinal growth data as indicators of seasonality in past environments. The sample consists of upper and lower incisors of fall-harvested beavers and upper incisors of winter-harvested beavers with known dates of death. All distances between sequential periradicular bands were measured using a dissecting microscope fitted with an ocular micrometer. All increments for each individual were matched to corresponding dates of formation. Records of daily temperatures, precipitation, and photoperiod corresponding to the locality and period of growth represented by the teeth were obtained from the National Weather Service. Time series for both dental increments and temperature were smoothed using nonparametric regression to partition "primary" (seasonal or long-term) and "secondary" (daily or short-term) signals. Upper and lower incisors differ in their primary signals in that uppers exhibit a marked decline in growth rate during the late summer and early fall, leveling off in the late fall. This trend roughly parallels seasonal decreases in temperature, precipitation, and photoperiod, although the decline in growth rate begins while the weather is still warm (in late summer). In contrast, lower incisors show only a very weak seasonal trend in growth rate. Analysis of residuals from the trend lines reveals only a weak association between daily growth rate and both daily temperature changes and precipitation. The results suggest that longitudinal growth rates in upper incisors may be used to estimate season of death in fossil rodents.